Dbstl miscellaneous notes

Special notes about trivial methods

There are some standard STL methods which are meaningless in dbstl, but they are kept in
dbstl as no-ops so as to stay consistent with the standard. These are:

db_vecter::reserve();

db_vector::max_size();

db_vector::capacity();

db_map::reserve();

db_map::max_size();

db_vector<>::max_size() and
db_map<>::max_size() both return 2^30. This does not mean
that Berkeley DB can only hold that much data. This value is returned to conform to some
compilers' overflow rules — if we set bigger numbers like 2^32 or 2^31, some compilers
complain that the number has overflowed.

See the Berkeley DB documentation for information about limitations on how much data a
database can store.

There are also some read-only functions. You set the configuration
for these using the Berkeley DB API. You access them using the container's methods. Again,
this is to keep consistent with C++ standard STL containers, such as:

db_map::key_comp();

db_map::value_comp();

db_map::hash_funct();

db_map::key_eq();

Using correct container and iterator public types

All public types defined by the C++ STL specification are present in dbstl. One thing to
note is the value_type. dbstl defines the value_type for each iterator and container class to be the raw
type without the ElementRef/ElementHolder
wrapper, so this type of variable can not be used to store data in a database. There is a
value_type_wrap type for each container and iterator type,
with the raw type wrapped by the
ElementRef/ElementHolder.

For example, when type int_vector_t is defined as

db_vector<int, ElementHolder<int> >

its value_type is int, its
value_type_wrap is ElementHolder<int>,
and its reference and pointer types are ElementHolder<int>& and
ElementHolder<int>* respectively. If you need to store data, use
value_type_wrap to make use of the wrapper to store data
into database.

The reason we leave value_type as the raw type is that we
want the existing algorithms in the STL library to work with dbstl because we have seen that
without doing so, a few tests will fail.

You need to use the same type as the return type of the data element retrieval functions to
hold a value in order to properly manipulate the data element. For example, when calling